Polyurethane compositions based on the reaction product of a polyol and a polyisocyanate cured with one or more polyfunctional crosslinking agents have been described in the art. In particular, polyurethanes based on prepolymers comprising the reaction product of long chain fatty acid esters such as castor oil with organic polyisocyanates have been described. For example, U.S. Pat. No. 3,362,921 shows prepolymers based on the reaction product of active hydrogen-containing compounds such as castor oil, polyester amides and polyalkylene ether glycols with organic diisocyanates. Curing agents employed with these prepolymers are esters of polyhydric alcohols which contain at least four hydroxy groups and an aliphatic acid of at least 12 carbon atoms and one or more hydroxy and/or epoxy groups. The cured polyurethanes of U.S. Pat. No. 3,362,911 find use as flocking adhesives, paper coatings, potting compositions and encapsulating compounds for electronic parts.
U.S. Pat. No. 3,483,150 also discloses prepolymer compositions. These compositions are the reaction product of at least one polyfunctional compound that contains active hydrogens with an arylene diisocyanate, and a low viscosity or solid polyfunctional isocyanate derived from the reaction of aniline and a formaldehyde. The prepolymers are cured to elastomers by employing at least one curing agent that contains two or more active hydrogen groups. Such curing agents include the curing agent of U.S. Pat. No. 3,362,921 and in addition, a glycol, glycerol, polyglycol, or polyalkylene glycol mono- or di-ester of a hydroxy carboxylic acid having at least 12 carbon atoms. Amines such as primary and secondary aliphatic, cyclic, aromatic, aralkyl and alkaryl diamines are useful in curing these prepolymers.
Cured polyurethanes have found use in hollow fiber separatory devices. U.S. Pat. No. 3,962,094 shows a hollow fiber separatory device useful for dialysis, ultra-filtration, reverse osmosis, hemodialysis, etc. This device consists of a plurality of fine, hollow fibers whose end portions are potted in a tube sheet and whose open fiber ends terminate in a tube sheet face which provides liquid access to the interior of the fibers. The tube sheet comprises a cured polyurethane consisting essentially of a prepolymer based on the reaction product of castor oil with at least one mole per castor oil of an organic diisocyanate and cross-linked with either castor oil or an ester of a polyhydric alcohol having a hydroxyl functionality of 4 or more, and an organic acid containing at least 12 carbon atoms and one or more hydroxy and/or epoxy groups per molecule, or mixtures of castor oil and the such esters.
In addition to separatory devices, cured polyurethanes also have been employed in folded membrane separatory devices for use in chemical separations such as dialysis, osmotic processes and hemodialysis. In a folded membrane device such as an artificial kidney, a membrane sheet is multiply-folded or pleated to form a series of adjacent channels, each channel located between opposed faces of each fold. The edges of the folds in the membrane are sealed together by potting the edges with a sealant. The membrane then is placed in a container such as polystyrene, a styrene-acrylonitrile copolymer or a polycarbonate polymer wherein the chemical separation takes place. In the case of dialysis, the dialysis solution is placed on one side of the membrane and blood is placed on the other side.
Polyurethane compositions which contain a polyol also are shown in U.S. Pat. No. 4,865,735. The polyurethanes of U.S. Pat. No. 4,865,735 are formed as the reaction product of an organic isocyanate, and a polyol of a carboxylic acid that has at least 8 carbon atoms. The urethanes of U.S. Pat. No. 4,865,735, although useful as potting materials, must be formulated as three part systems because the amine may cause the polyol to become unstable. Formulating three part polyurethanes, however, is both inefficient and expensive.
The three part polyurethane systems of the prior art, although useful as potting compounds such as for hollow fibers and folded membrane separatory devices, have additional drawbacks after the amine and polyol components have been mixed in preparation for urethane formation. The polyol becomes unstable, and potting results in inferior sealant performance. This performance problem can usually result in the failure of the separation device.
A need therefor exists for polyurethanes which may be employed as potting compounds and the like, such as in the manufacture of hollow fiber separation devices, but which avoid the requirement to formulate polyurethanes as three part systems.